The invention relates to a luminescent rare earth metal phosphate, which is activated by terbium. The invention also relates to a luminescent screen provided with such a phosphate and to a low-pressure mercury vapour discharge lamp provided with such a screen.
Tb.sup.3+ -activated phosphates of rare earth metals, notably those of Y and/or La are known to be very efficient luminescent materials. See, for example, U.S. Pat. Nos. 3,634,282; 3,403,279 and 3,211,666. The materials described in these United States Patents have the orthophosphate of La and/or Y, for example, LaPO.sub.4 as the fundamental lattice.
A drawback of these known luminescent phosphates is that they have to be activated not only by Tb but also by Ce. In fact, the Ce is necessary to absorb the exciting radiation, whereafter the excitation energy is transferred to the Tb. Therefore it is unavoidable that these known phosphates generally also exhibit short-wave Ce emission which is unusable.
The object of the invention is to provide novel efficiently luminescent materials in which Ce or another ion which can absorb the exciting radiation is not necessary, so that the emitted radiation is a pure Tb.sup.3+ emission.